The Organisation of Islamic Cooperation joins African, Gulf and Asian states in taking a strong stance against Gulenists, who were implicated in the July 15 failed coup attempt in Turkey.

Cavusoglu said that designating FETO as a terror group is a milestone for the OIC.

The Organisation of Islamic Cooperation (OIC) has joined Turkey in designating Gulenists as the Fethuallist Terrorist Organisation (FETO), Turkish Foreign Minister Mevlut Cavusoglu said on his official Twitter account on Wednesday.

Turkey's government accuses the organisation, headed by US-based cleric Fethullah Gulen, of organising the July 15 coup attempt. The failed coup resulted in hundreds of people being killed and thousands of others wounded.

"We thank the OIC for showing solidarity by understanding our sensitivities and addressing FETO as a terror organisation," Cavusoglu said.

Many African and Asian countries have lent their support to Turkey in its fight against the organisation.

The OIC is an international organisation founded in 1969 which aims to encourage cooperation and solidarity between countries with Muslim populations. It has 57 member states.

Gulen has repeatedly denied the accusations made against him. He said that the failed coup – during which dozens of civilians were killed by pro-coup soldiers – looked "like a Hollywood movie."

Fethullah Gulen has lived in self-imposed exile in the US state of Pennsylvania since 1999. (AP)

The OIC's decision comes on the same day as Ankara extended its state of emergency for another 90 days. Following the attempted coup, Turkey's government declared a state of emergency to tackle the threat posed by FETO.